This invention generally relates to blow molding machines and more particularly, to an extrusion head having a rotational adjustment for aligning a view stripe in a parison so as to correspond with the mold parting line on a blown container.
Plastic containers, such as motor oil containers, are generally made of an opaque and colored plastic resin. The color of the container generally provides product identification to the consumers. A disadvantage of these opaque containers is that, during use, it is not possible for the consumer to visually monitor the level of the contents remaining in the container. One solution for ascertaining the remaining content amount is to form a vertical stripe, made of a translucent plastic resin, in the side wall of the container through which the content level can be viewed. These vertical stripes are more commonly known as view stripes.
Typically, the container is round, square or rectangular and is provided with a view stripe that corresponds with or is formed on the mold parting line of the blown container. One benefit of locating the view stripe on the parting line of a generally rectangular or square container is that it enables the container to be manufactured with the view stripe confined to the narrow side walls. This also avoids having the view stripe extend through the front and back side walls, which may carry labels for product identification, and through the top and bottom of the container where, in many cases, the view stripe is useless. Regardless of whether the container is rectangular or round, the view stripe must be positioned so that it is not formed into the volume level markings on the surface of the mold cavity.
U.S. Pat. No. 4,802,833, commonly assigned to the assignee of the present invention, discloses an accumulator head in a blow molding machine for producing plastic storage drums having vertical view stripes in their side walls. U.S. Pat. No. 4,890,994, also commonly assigned to the assignee of the present invention, discloses an apparatus for forming a view stripe in a parison and in which the view stripe is restricted to the side wall of the blown container. These prior patents tend to be directed toward intermittent extrusion processes. The present invention, however, is mainly directed toward use with continuous extrusion machines.
While the view stripe is originally aligned so that it will correspond with the mold parting line of the container, certain mold styles, in particular the book style molds that are used with certain continuous wheel style extrusion blow molding machines, have been found to cause the parison and the view stripe to rotate during the non-symmetrical closing of the mold halves prior to blow molding. Unfortunately, when designing blow molding machines it is practically impossible to predict the amount of rotation which will be experienced by the parison during the closing of the molds. Furthermore, changing of the molds themselves and molding a different container will often result in a change in the amount of parison rotation that occurs during closing of the mold. Regardless of how the rotation occurs, when the rotated parison is blow molded, the position of the view stripe will deviate from the mold parting line of the blown container. If the degree of departure from the mold parting line is great enough, the resulting view stripe will not only be formed off of the mold parting line, but will also be formed over the volume level markings or indicia on the container side wall. Additionally, if the view stripe is formed off of the mold parting line, the view stripe will exhibit a visible "bend" as it passes from the side wall into the upper and lower container walls. For aesthetic reasons, it can be seen that it is desirable to maintain the view stripe restricted to the side walls and particularly the mold parting line thereby preventing the view stripe from being formed over the volume level markings or deviating from a straight line as it breaks onto the top/bottom walls.
It is therefore an object of this invention to provide an apparatus for maintaining the position of the view stripe on the mold parting line of the blown container. It is a further object of this invention to provide an apparatus which has the ability to adjust the position of the view stripe, relative to the mold halves, being formed in a parison to accommodate shifting and rotation of the parison during non-symmetrical closing of the mold halves.
A further object of this invention is to accomplish the above objects while eliminating stagnation pockets in the flow of the plastic resin which would result in degregation of the plastic forming the parison. Typically, stagnation pockets form in corners, dead ends and abrupt direction changes. One challenge which needed to be overcome by this invention was to provide the above mentioned adjustability while streamlining the flow of resin and eliminating substantially all areas where stagnation pockets could develop, including corners, dead ends and abrupt direction changes in the adjustment mechanism itself.
In achieving the above objects, the apparatus of this invention is typically utilized in a single cavity extrusion head assembly. However, a multi-cavity assembly might alternatively be employed. The parison is formed from two plastic resins, a primary resin and a secondary resin, with the view stripe longitudinally extending in the wall of the parison. The primary resin is pigmented and opaque. The secondary resin is unpigmented and translucent, but often is of the same or a similar type of resin as the primary resin.
While in a molten state, the primary and secondary resins are individually fed into the interior cavity of the extrusion head. The flow of the primary resin is divided into two flow streams which progress in opposite directions around a mandrel sleeve, positioned within the primary cavity of the extrusion head, and are then rejoined. A diverter, formed on the mandrel sleeve, directs the flow stream from a generally horizontal direction to a generally downward direction. The primary resin flow moves through length of the cavity sufficient to eliminate substantially all aspects of rotational flow thereby leaving only an axial component to the flow direction. As the primary resin moves downward, a vertical division is caused to be formed in the primary resin flow.
The secondary, unpigmented resin is fed into the extrusion head and is first directed into a circular flow passage located on the adjustment ring of the assembly. At the entrance of the circular flow passage, the secondary resin is divided into two flows which proceed around opposite sides of the circular flow passage until being rejoined and directed downward into a flow passage having a exit port. The exit port is circumferentially adjustable by virtue of it being formed in the rotatable adjustment ring. The exit port of the circular flow passage corresponds with the division in the primary resin flow and results in the secondary, unpigmented resin being inserted into the division of the primary resin flow. The opposing edges of secondary resin thus form welds joining with the divided portions of the primary resin flow.
As a result, a tubular parison is formed having an axial view stripe of a translucent resin in the wall of opaque resin. The circumferential position of the view stripe in the wall of the parison ca be readily altered by the adjustment ring to compensate for rotation or shifting of the parison caused during closing of the mold halves for blow molding. The actual mechanism for achieving this adjustment is provided externally of the extrusion head for convenience and ease of operation.
The formed tubular parison is extruded through an annular outlet in the bottom of the extrusion head. The mold halves of the blow molding machine are positioned below the parison outlet such that the mold halves can be closed upon the parison. By providing the extrusion head with a rotationally adjustable exit port for the secondary resin, the position of the view stripe in the parison can be varied to compensate for any rotation or shifting caused by the mold halves and thereby ensure correspondence of the view stripe with the mold parting line or any otherwise desirable location. As will be appreciated by the discussion more fully set out below, the specific structure of the present invention allows for positioning of view stripe to be accomplished without any significant degregation of the plastic resin occurring. The reduced degregation is attributable to the fact that while the secondary resin flow is divided into two streams, movement of each of the divided streams is continuous with the streams later being rejoined. This continuous flow is provided by the circular flow passage that allows the resin to flow around the left or right arm of the passage, without disruption, to the exit port. This is advantageous to an arcuate, but non-circular or non-closed loop, flow passage in which the inlet port is of a fixed location while the passage and the exit port are arcuately adjustable to vary the position of the view stripe. In the non-closed loop arrangement, if the exit port is adjusted and moved generally toward the inlet port, a portion of the arcuate passage will be caused to extend beyond the inlet port and begin to form a dead end passage on the side of the inlet port opposite the exit port. During use, the secondary resin would then flow into this dead end passage, stagnate and degregate.
A container formed according to the present invention will include a vertical stripe of a translucent resin extending along one side wall of the container, generally along the mold parting line formed on the container during blow molding. By positioning the view stripe on the mold parting line, a portion of the view stripe will end up as flash extending above and below the side wall. After blow molding, the flash is removed and the view stripe will conveniently end adjacent to the upper and lower ends of the side wall.
Any plastic resin which is suitable blow molding, and which can be utilized in either an opaque or translucent form, is within the purview of this invention. In a container produced by this invention, the opaque and translucent plastic resins must be compatible for joining together at the weld lines. In one of its forms, high density polyethylene, for both the opaque and translucent resins, produces a container with a view stripe through which the content level within the container can be readily ascertained.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.